When the equation is of the form
... ax² +bx +c = 0
the discriminant is
... d = b² -4ac
Here, your equation is in the appropriate form, so we can identify
... a = -1, b = 4, c = -2
and calculate the discriminant as
... d = 4² - 4(-1)(-2) = 16 - 8 = 8
The discriminant is 8.